246 ACTION OP FOEESTS ON THE PLOW OP BIVERS. 



evil is avoided ; and, what is of more importance still, the forest acts 

 at the same time as a filter, delivers no water but what is of perfect 

 fluidity, scarcely even discoloured by the washing away of organic 

 matter, and unable to wash away the earth of the subsoil protected 

 against erosion by its thick covering of humus. 



" When, on the contrary, the rain falls on a soil stript of vegetation, 

 it tends to cut this up into ravines, and it does so if the tenacity and 

 resistance of the ground be not suflELcient to withstand it ; and the 

 flood is subject to great variations in its current, carrying oS, here 

 and there the earth and other debris of the soil. 



" Forests have, then, a double action ; on the one hand they 

 consolidate the soil, on the other hand they reduce and regulate the 

 flow of the current, — acting at once both on the delivery and on the 

 perturbation, — in other words, on the primary cause and on the 

 secondary causes of the overflowing of water-courses. 



" It has been tried to subject to experiment and observation the 

 meteorological and hydrological efi'ects of forests. And doubtless 

 studies so interesting are by no means lost to science. They cannot 

 be too much encouraged ; but it should be borne in mind that they 

 can have comparatively little value in this question, seeing that they 

 cannot take cognisance of this modulating and regulating action. 



" In regard to any flood which we may wish to make the subject of 

 study, it would avail comparatively little to know what quantity of 

 rain falls annually in the basin drained by it. What is necessary to 

 be known is — -In what way did the flow of the flood operate during 

 the duration of the flood, taking into account the quantity of water 

 discharged, and all the causes or sources of perturbation operating — 

 which is a much more difficult problem. 



" And in resolving the whole question into the permeability of the 

 soil, and its capacity of absorption, it appears importance is attached 

 exclusively to the reduction of the volume of water which floWs 

 away. It seems to be forgotten in this that water-courses, if steadily 

 supplied, constitute it may be said the principal riches of a country, 

 and the most potent of all instruments of labour. 



" By their modulating power forests act as vast reservoirs, not only 

 in preventing sudden variations of delivery during a flood, but in 

 feeding the water-courses and raising their level during the period of 

 exhaustion. In what relates specially to the torrents of the Alps 

 it has been demonstrated that the reueweddevastating power which 

 they have exhibited, and which has assumed such portentous mao'ni- 

 tude in the course of the last forty years is a consequence of the dis- 



